| Sıra | DOSYA ADI | Format | Bağlantı |
|---|---|---|---|
| 01. | Slicing Thing Search Returns | ppt | Sunumu İndir |
Transkript
Chapter 4Computing With StringsCharles SeveranceTextbook: Python Programming: An Introduction to Computer Science, John Zelle
String Data Type•A string is a sequence of characters•A string literal uses quotes ‘Hello’ or “Hello”•For strings, + means “concatenate”•When a string contains numbers, it is still a string•We can convert numbers in a string into a number using int()>>> str1 = \Hello“>>> str2 = 'there‘>>> bob = str1 + str2>>> print bobHellothere>>> str3 = '123‘>>> str3 = str3 + 1Traceback (most recent call last): File \<stdin>\, line 1, in <module>TypeError: cannot concatenate 'str' and 'int' objects>>> x = int(str3) + 1>>> print x124>>> Z-78 Z-98
Input() is kind of useless•When using input(“Prompt”) it is actually looking for an expression from input•We use this just to prompt for numbers for simple programs•We use raw_input(“Prompt”) for non-trivial programs>>> x = input(\Enter \)Enter helloTraceback (most recent call last): File \<stdin>\, line 1, in <module> File \<string>\, line 1, in <module>NameError: name 'hello' is not defined>>> x = input(\Enter \)Enter 2 + 5>>> print x7>>> Z-78
Real ProgramsUse String Input•We prefer to read data in using strings and then parse and convert the data as we need•This gives us more control over error situations and/or bad user input•Raw input numbers must be converted from strings>>> name = raw_input(\Enter:\)Enter:Chuck>>> print nameChuck>>> apple = raw_input(\Enter:\)Enter:100>>> x = apple – 10Traceback (most recent call last): File \<stdin>\, line 1, in <module>TypeError: unsupported operand type(s) for -: 'str' and 'int‘>>> x = int(apple) – 10>>> print x90Z-79
What Kind of Thing?•We have a way to see what *kind* of data is in a variable•We use a special function called type() to look at the kind of data is in a variable>>> x = \Hello\>>> print xHello>>> print type(x)<type 'str'>>>> y = \Bob\>>> print yBob>>> print type(y)<type 'str'>>>> z = 45>>> print z45>>> print type(z)<type 'int'>>>>
Looking Inside Strings•We can get at every single character in a string using an index specified in square brackets•The index value can be an expression that is computed•The index value must be an integerZ-80
Slicing Strings•We can also look at any continuous section of a string using a colon•The second number is one beyond the end of the slice - “up to but not including”• If a number is omitted it is assumed to be the the beginning or end>>> greet = \Hello Bob“>>> greet[0:3]'Hel‘>>> greet[5:9]' Bob‘>>> greet[:5]'Hello‘>>> greet[5:]' Bob‘>>> greet[:]'Hello Bob' Z-81
String indexes from the right•Negative index numbers in a string start from the right (or end) of the string and work backwards>>> greet = \Hello Bob“>>> greet[-1]'b‘>>> greet[-3]'B'-1-2-3-4-5-6-7-8-9Z-80
A Character too Far•You will get a python error if you attempt to index beyond the end of a string.•So be careful when constructing index values and slices>>> zot = \abc“>>> print zot[5]Traceback (most recent call last): File \<stdin>\, line 1, in <module>IndexError: string index out of range>>>
String Operators•We do a lot of work with strings and Python has a lot of support for strings•With respect to strings, Python is a “smooth operator”Z-82
How Long is a String?•The len() function takes a string as a parameter and returns the number of characters in the string•Actually len() tells us the number of elements of any set or sequence>>> greet = \Hello Bob“>>> print len(greet)9>>> x = [ 1, 2, \fred\, 99]>>> print len(x)4>>> Z-82
Len Function>>> greet = \Hello Bob“>>> x = len(greet)>>> print x9len()function“Hello Bob” (a string)9(a number)A function is some stored code that we use. A function takes some input and produces an output.Guido wrote this code
Len Function>>> greet = \Hello Bob“>>> x = len(greet)>>> print x9def len(inp): blah blah for x in y: blah blah“Hello Bob” (a string)9(a number)A function is some stored code that we use. A function takes some input and produces an output.
Multiplying Strings?•While it is seldom useful, the asterisk operator applies to strings>>> zig = \Hi“>>> zag = zig * 3>>> print zagHiHiHi>>> x = “ “*80Z-81
Looping Through a String•A string is a sequence (ordered set) of characters•The for loop iterates through a sequence, with the iteration variable taking successive values from the sequence each time the loop body is run>>> zap = \Fred“>>> for xyz in zap:... print xyz... Fred>>> Z-96
String Library
String Library•Python has a number of string operations which are in the string library•We use these library operations quite often when we are pulling apart input data•To use these, we import the string library>>> import string>>> greet = \Hello Bob“>>> zap =string.lower(greet)>>> print zaphello bob>>> Z-96
What is a Library?•Some super developers in the Python world write the libraries for us to use• import string•Somewhere there is a file string.py with a bunch of def statementsstring.pydef split(inp): blah blahdef upper(inp): for i in blah: blahdef find(inp): blah blah
Z-96http://docs.python.org/lib/string-methods.html
Searching a String•We use the find() function to search for a substring within another string•find() finds the first occurance of the substring• If the substring is not found, find() returns -1•Remember that string position starts at zero>>> import string>>> greet = \Hello Bob\>>> pos = string.find(greet,\o\)>>> print pos4>>> aa = string.find(greet,\z\)>>> print aa-1Z-94-95
Making everything UPPER CASE•You can make a copy of a string in lower case or upper case•Often when we are searching for a string using find() - we first convert the string to lower case so we can find a string regardless of case>>> import string>>> greet = \Hello Bob\>>> nnn = string.upper(greet)>>> print nnnHELLO BOB>>> lll = string.lower(greet)>>> print lllhello bob>>> Z-94-95
Search and Replace•The replace() function is like a “search and replace” operation in a word processor• It replaces all occurrences of the search string with the replacement string>>> import string>>> greet = \Hello Bob\>>> nstr = string.replace(greet,\Bob\,\Jane\)>>> print nstrHello Jane>>> greet = \Hello Bob“>>> nstr = string.replace(greet,\o\,\X\)>>> print nstrHellX BXb>>> Z-94-95
Stripping Whitespace•Sometimes we want to take a string and remove whitespace at the beginning and/or end• lstrip() and rstrip() to the left and right only• strip() Removes both begin and ending whitespace>>> import string>>> greet = \ Hello Bob \>>> string.lstrip(greet)'Hello Bob '>>> string.rstrip(greet)' Hello Bob'>>> string.strip(greet)'Hello Bob'>>> Z-94-95
Breaking Strings into Parts•We are often presented with input that we need to break into pieces•We use the split() function to break a string into a sequence of strings>>> import string>>> abc = \With three words“>>> stuff = string.split(abc)>>> print stuff['With', 'three', 'words']>>>Z-92, Z-96
>>> import string>>> abc = \With three words“>>> stuff = string.split(abc)>>> print stuff['With', 'three', 'words']>>> print len(stuff)3>>> print stuff[1]threeZ-92, Z-96>>> print stuff['With', 'three', 'words']>>> for w in stuff:... print w...Withthreewords>>> Split breaks a string into parts produces a list of strings. We think of these as words. We can access a particular word or loop through all the words.
>>> import string>>> line = \first,second,third“>>> thing = string.split(line)>>> print thing['first,second,third']>>> print len(thing)1>>> thing = string.split(line,\,\)>>> print thing['first', 'second', 'third']>>> Z-92, Z-96>>> line = \A lot of spaces“>>> etc = line.split()>>> print etc['A', 'lot', 'of', 'spaces']>>> You can specify what delimiter character to use in the splitting. Also when you do not specify a delimiter, multiple spaces is thought of as “one” delimiter.You can also just add .split() to the end of a string variable.
File Processing
File Processing•A text file can be thought of as a sequence of linesFrom stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008Return-Path: <postmaster@collab.sakaiproject.org>Date: Sat, 5 Jan 2008 09:12:18 -0500To: source@collab.sakaiproject.orgFrom: stephen.marquard@uct.ac.zaSubject: [sakai] svn commit: r39772 - content/branches/Details: http://source.sakaiproject.org/viewsvn/?view=rev&rev=39772Z-107
Opening a File•Before we can read the contents of the file we must tell Python which file we are going to work with and what we will be doing with the file•This is done with the open() function•open() returns a “file handle” - a variable used to perform operations on the file•Kind of like “File -> Open” in a Word ProcessorZ-108
Using open()•handle = open(filename, mode)• returns a handle use to manipulate the file•filename is a string•mode is “r” if we are planning reading the file and “w” if we are going to write to the file.http://docs.python.org/lib/built-in-funcs.htmlfhand = open(\mbox.txt\, \r\)Z-108
File Handle as a Sequence•A file handle open for read can be treated as a sequence of strings where each line in the file is a string in the sequence•We can use the for statement to iterate through a sequence•Remember - a sequence is an ordered setxfile = open(\mbox.txt\, \r\)for cheese in xfile: print cheese
Counting Lines in a File•Open a file read-only•Use a for loop to read each line•Count the lines and print out the number of linespizza = open(\mbox.txt\, \r\)howmany = 0for slice in pizza: howmany = howmany + 1print howmanyZ-107, Z-110
Summary• String Data Type• input() and raw_input()• Indexing strings• Slicing strings• String operators• String len() function• Looping through a string• String Library• Searching strings• Changing Case• Removing Whitespace• Splitting a string into parts• File Processing• Opening a File• Looping through a file
Slicing Thing Search Returns
Yıl : 2020
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